This invention relates to programmable logic array devices having embedded random access memory arrays which can be configured as programmable product-term-type logic elements if desired. More particularly, the invention relates to programmable logic devices having look-up tables for performing logic and larger blocks of random access memory which are usable by the user for such purposes as data storage and additional look-up table logic, and which larger blocks of random access memory are alternatively configurable as programmable product-term-type logic elements.
One known type of programmable logic device includes an array of programmable AND gates which typically produces multiple outputs, each generally resulting from the ANDing of multiple inputs. These AND gate array outputs are commonly referred to as "product terms" because the logical representation of the AND function is analogous to multiplication. Generally, a plurality of these product terms, or "p-terms," are combined by an OR gate to produce a sum-of-products output (the OR function being analogous to addition).
Another type of programmable logic device is implemented using many relatively small look-up tables whose inputs are either the inputs of the programmable logic device or the outputs of other look-up tables in the device.
Programmable logic architectures have recently been developed in which relatively large, user-configurable blocks of random access memory (RAM) are provided among blocks of look-up-table-type programmable logic. One such architecture is described in Cliff et al. U.S. Pat. No. 5,689,195, which is hereby incorporated by reference herein in its entirety. These user-configurable memory blocks can be used as general-purpose memory for the device, or they can be used as additional relatively large look-up-table-type logic blocks.
Look-up-table-type logic may have a disadvantage relative to p-term-type logic with respect to the number of inputs to a logic function that can be implemented in one reasonably sized block of circuitry. For example, the above-mentioned Cliff et al. reference shows devices having many four-input look-up tables and several relatively large blocks of user-configurable RAM that can function as eight- to 11-input look-up tables. To perform logic functions of more than 11 inputs in such a device it is necessary to use a tree of the available look-up table units. It is not practical to redesign devices of this kind with larger user-RAM blocks to individually act as look-up tables having significantly larger numbers of inputs (e.g., 20, 30, or more inputs) because such RAM blocks would have to be extremely large. However, p-term-type logic arrays with 20, 30, or even more inputs are not excessively large and can therefore more readily provide outputs which are functions of large numbers of inputs.
In view of the foregoing, it is an object of this invention to provide look-up-table-type programmable logic devices with the capability of more readily performing some logic functions having large numbers of inputs.
It is another object of this invention to provide look-up-table-type programmable logic devices which include relatively large blocks of user-configurable RAM with the capability of optionally performing some logic functions using p-term-type logic in the user-configurable RAM if desired.